1. Field of the Invention
The present invention relates to a hybrid vehicle of the type driven by combination of an internal combustion engine (hereinafter referred to as "engine") and an electric motor (hereinafter referred to as "motor").
2. Description of the Prior Art
In one type of the conventional hybrid vehicles, an engine and a motor are connected to each other. This type has three drive modes, the first being operated only by the engine, the second being operated only by the motor and the third being operated by combination of the engine and the motor.
Another type of the conventional hybrid vehicles includes an engine and an electric generator connected to each other so that a part of the output energy from the engine is transmitted to the generator. The remainder of the output energy is transmitted directly to an output drive shaft. The hybrid vehicle of this type provides an improved fuel consumption because the engine may be run at peak efficiency with a controlled reaction force of the generator and all of the output energy from the engine may be utilized for generation of electricity.
However, in such conventional hybrid vehicles, the generator's revolution is increased to an extraordinary high level when it becomes uncontrollable due to accident to a control system of the generator, cutoff of a line from a power source, short circuit of a stator coil, etc. This could be prevented by providing a sensor that detects the generator's revolution and a control unit that operates when the generator's revolution exceeds a predetermined one to enhance the electricity generation to thereby absorb the engine torque. However, this solution requires an oversize generator, which inevitably increases the overall weight of the hybrid vehicle.